This invention is directed to modifying scrap tire rubber for use in or as other beneficial and desirable products.
Almost 3 million tons of scrap tires are annually generated in North America. Among the rubber recycling approaches, size reduction is a feasible approach for the most beneficial reuse of these materials. Currently, there are limited applications for the size reduced rubber particles, and development of new products is essential to expanding useful applications. One restricting factor on reusing rubber particles is the hydrophobic nature that limits the use to only non-aqueous media.
Polymers are generally classified as thermoplastics and thermosets. Thermoplastic polymers can generally be melted and recycled using heating and remolding processes. Thermoset polymers are crosslinked and, once they are set, the simple approach of melting and reforming into a new shape does not apply to them. Recycling of thermoset polymers is thus a challenging technical problem. Vulcanized rubber materials are thermoset polymers, and scrap tires represent the largest stream of waste rubber materials. Over 250 million tires (about one tire per person) are annually generated in North America and, for the most part, these are inefficiently used or disposed of in landfills.
More attention has been focused on reusing scrap tires in the past few years, but the current applications are generally limited to “low value” applications. Understanding the present markets for scrap tires is a key to continuation and expansion of the recycling efforts toward higher value added uses of these materials. Currently, there are typically three major markets for scrap tires: tire derived fuel (TDF); civil engineering applications; and crumb rubber applications. In addition, small percentages of scrap tires used are often exported or used in agricultural applications.
Incineration of scrap tires to generate energy is a well-known technology and is the largest market for scrap tires in North America. TDF as a source of energy is probably as efficient, and possibly less expensive, than fossil fuels. As late as 1990, the only recycling approach for scrap tires was the use of TDF. More environmentally friendly applications have been developed since then, such that now only about 50 percent of the total recycled scrap tires are in TDF applications.
TDF is a sufficient way for reducing the number of stockpiled scrap tires. However, a valuable source of raw materials is lost using this approach. Possible reuse of scrap tires in new products provides considerably more energy than simply burning.
Civil engineering is a fast growing and the second largest market for scrap tires. In the typical civil engineering application, shredded tire is used where there is an economic benefit as compared to the price of soil or other fill materials. The two major factors contributing to the dynamic growth of this market are the existence of a considerable amount of tire shreds from stockpile abatement projects and availability of significant guidelines and information for shredded tire in civil engineering applications. Civil engineering applications are not considered high value added uses of scrap tires, because in most applications the rubber particles are used as a replacement for generally inexpensive materials like soil.
Size reduction seems to be a promising recycling approach for beneficial uses of scrap tires in high performance products. Size reduction generally refers to grinding the vulcanized rubber into shredded particles in the typical size range of 25 mm to 150 mm. Further size reduction of the shredded materials into smaller particles (less than 2000 microns) is defined as pulverization. According to ASTM D 5603-96, the recycled rubber particles are classified as coarse and fine particles. Rubber particles in the size range of 2000 microns to 425 microns are coarse particles and the particles smaller than 425 microns are classified as fine particles.
Today, there are four major applications of crumb rubber usage: rubber modified asphalt (RMA), molded products, tire/automotive industry and sport surfacing. These applications could be considered as higher value added use of recycled rubber materials, compared to the civil engineering and TDF applications discussed above.
The crumb rubber applications have a limited market compared to the other applications. An important factor impacting this market is the imbalance in supply and demand of crumb rubber materials. To increase demand, there is a need for more focus on developing more products for new applications.
Direct addition of rubber particles into the matrix of another polymer that is incompatible with rubber particles generally results in poor mechanical properties of the produced materials. Poor interfacial adhesion between surfaces of the rubber particles with matrix is typically the main reason for these failures. Surface modification of the rubber particles, or addition of a compatiblizer, may enhance the mechanical properties of the resulting composite materials. The molded product of the binder and rubber particles might have limited application due to the particular shape of the mold used. A broader range of applications could be obtained without using a mold if the binder is also in the particulate form. The potential application of such composite materials is in polymeric surface coatings such as waterborne polymeric coatings or dry powder coatings. Waterborne polymeric emulsions are the most suitable choice of binders in particulate form. However, recycled rubber particles have a very poor dispersibility in aqueous media due to their hydrophobic nature. Addition of a hydrophilic character to hydrophobic rubber particles would allow their utilization in such media.
There is a need for a method of modifying reclaimed rubber particles from scrap tires to make them more useable in higher value applications and products. There is a need for a modified rubber particle that has hydrophilic properties.